The present invention relates to an apparatus for controlling a hydraulic elevator in which a hydraulic pump is driven by a variable-speed electric motor to send pressurized oil to a cylinder, in order to run the cage.
Conventional systems for controlling the hydraulic pressure of a hydraulic elevator can be divided into a system based upon a control valve for controlling the flow rate, a system for controlling the pump, and a system for controlling the number of revolutions of an electric motor.
According to the system based upon a control valve to control the flow rate, the electric motor is rotated at a constant speed when the elevator is to be raised while permitting the oil from a hydraulic pump to return to the tank at a predetermined pumping rate. When a start instruction is produced, the flow rate of oil returning into the tank is adjusted by the flow-rate control valve to control the speed of the cage. Further, when the elevator is to be lowered, the descending speed of the cage due to its own weight is adjusted by the flow-rate control valve, in order to control the speed of the cage. According to this system, however, the oil is circulated wastefully when the cage is to be raised, and the potential energy of the cage is consumed for heating the oil when it is to be lowered. Therefore, the energy loss is great, and the temperature of the oil is raised remarkably.
To compensate for these defects, there has been proposed a system in which the oil is supplied in a required amount only when the elevator is to be raised, and regenerative braking is applied to the electric motor when the elevator is to be lowered. This type system can be divided into one which controls the pump, and one which controls the number of revolutions of the electric motor. With the system which controls the pump, use is made of a pump of the variable-capacity-type, and the pumping rate of the pump is adjusted by a control device, resulting, however, in complex construction of the control device and pump, as well as expensive manufacturing cost.
Accompanying the technical progress of semiconductor devices in recent years, on the other hand, there has been proposed a system for controlling the number of revolutions of an induction motor over a wide range by varying the voltage and frequency, as has been disclosed, for instance, in Japanese Patent Laid-Open No. 98477/1982. Based upon this system is a system for controlling the number of revolutions of an electric motor, according to which use is made of a pump of the type having a constant pumping rate, and the pumping rate is controlled by changing the running speed of the electric motor. Therefore, this system can be cheaply manufactured and is highly reliable.
That is, when the elevator is to be raised, the hydraulic pump is driven by the electric motor to feed the pressurized oil into a cylinder. When the elevator is to be lowered, on the other hand, the hydraulic pump is rotated by the pressurized oil to drive the electric motor, thereby to regenerate the electric power.
In practice, however, leakage is inherent in the hydraulic pumps. Due to the leakage, therefore, there exists a range in which the cage does not move even when the hydraulic pump is rotated. That is, as shown in FIG. 1, a start instruction may be produced at a time t.sub.O. The hydraulic pump is gradually accelerated and reaches a number of revolution n.sub.1 at a time t.sub.1. As the number of revolutions becomes greater than n.sub.1, the hydraulic pump pumps the oil at a rate greater than the leakage rate, and the cage starts to move. Thus, if the number of revolutions is rapidly increased, the oil is supplied to a pipe between the hydraulic pump and a check valve at a rate greater than the leakage rate. Therefore, a high pressure is generated to rapidly push the check valve open, causing a large starting shock and vibrations to be generated. The cage reaches a constant speed at a time t.sub.2, starts to decelerate at a time t.sub.3, and comes into halt at a time t.sub.4. However, the hydraulic pump continues to rotate, and comes to a halt at a time t.sub.5. The starting shock stems chiefly from a sudden increase in the number of revolutions of the hydraulic pump. Therefore, if the number of revolutions is more gradually increased as shown in FIG. 2, the cage starts to move at a time t.sub.11, reaches a constant speed at a time t.sub.12, starts to decelerate at a time t.sub.13, and stops at a time t.sub.14. Thereafter, the hydraulic pump stops at a time t.sub.15. Accordingly, if the number of revolutions is more gradually increased as mentioned above, the shock can be reduced. However, the delay in the start mode of operation increases giving the passengers the impression of sluggish operation. Further, the operation time is lengthened, whereby the transport efficiency is decreased.
The descending mode of operation of the hydraulic elevator is further described below with reference to FIG. 3. Production of a start instruction at a time t.sub.O causes the check valve to open. Therefore, the pressurized oil is allowed to flow suddenly into a space between the check valve and the hydraulic pump. As this space is filled with the oil, the hydraulic pump interrupts the flow of pressurized oil, and the flow of oil decreases rapidly from the clinder. Therefore, at the start of the descent operation, the cage descends rapidly but is stopped immediately thereafter causing the cage to vibrate as shown in FIG. 3(c). The cage increases its speed accompanying the increase in the number of revolutions of the hydraulic pump, while being vibrated, and reaches a constant speed. When a deceleration instruction is issued at a time t.sub.1, the hydraulic pump decelerates. Accompanying the deceleration, the cage also decelerates, and assumes a small constant speed. When a stop instruction is issued at a time t.sub.2, the hydraulic pump stops at a time t.sub.3. The cage continues to descend at a very small speed corresponding to the leakage amount of the hydraulic pump, and comes to a halt while being vibrated when the check valve is closed at a time t.sub.4.
As described above, the hydraulic elevator generates large vibration when it starts and stops, whereby the comfort of passengers is reduced.